Disk drives, such as hard disk drives, floppy disk drives, CD-ROM drives, etc., are widely used in computers and other devices for data storage. Such disk drives typically include a magnetic or optical disk upon which data is stored. Data is read from and/or written to the disk by a magnetic or optical read/write head (or heads) that is movable with respect to the disk and is positioned by a motor based upon positioning information provided by the computer.
One common motor used in disk drives includes a “voice coil” that serves as an actuator to move and position the read/write heads. The name voice coil comes from the similarity of such coils to those commonly used in audio loudspeaker systems. That is, such voice coils are operated in a manner similar to acoustic voice coils in that a positive current applied to the coil produces a corresponding positive direction of motion of the read/write head, and a negative current applied to the voice coil produces a corresponding negative displacement of the read/write head.
Because of the high degree of accuracy and extremely fast speeds required of disk drives, high precision components are typically used in such drives. For the same reasons, it is also typically necessary to determine any error or deviation in the drive current of the voice coil. This error may then be used by the driving stage for the motor to make the appropriate adjustments as necessary to ensure that proper positioning of the read/write head is maintained at all times. Such error signal may be generated using an operational amplifier connected in a feedback loop from the voice coil to a driving stage of the motor, for example.
When designing such control feedback loops, it may be desirable to achieve a fast transient response so that none of the active elements in the control loop enter saturation. This may be especially true for the driving stage of a voice coil. Once an amplifier or driving stage becomes saturated, response time may be severely affected due to the delay required to get out of saturation.
To prevent such operational amplifiers and driver stages from saturating, it may be necessary to limit the swing of input and/or output signals thereof. One prior art error detection circuit 100 for use with a voice coil motor which provides output signal clamping is now described with reference to FIG. 3. The circuit 100 includes an error amplifier 101, which is a typical operational amplifier configured as an inverting amplifier having a gain of 10 by way of the resistors 102 (having a value of 10R) and 103 (having a value of R). The error amplifier 101 is used to determine the voltage difference between a known reference or control signal and a feedback signal from the voice coil feedback loop that provides information about the voice coil motor current. The difference in voltage between these two signals is the error signal. This error signal is used to control the conduction of the output drivers to control the voice coil current.
In an effort to ensure that the output stage of the error amplifier 101 does not saturate, the output is clamped to define the upper and lower boundaries of the output voltage. To this end, four diodes 103-106 are connected in the error amplifier 101 feedback loop to limit or clamp the output swing. In operation, if the output of the error amplifier 101 is driven high, both diodes 103 and 104 conduct when the output voltage exceeds two diode drops (approximately 1.4 V) above the reference voltage potential, or 5 V in this example. Similarly, if the output of the error amplifier 101 is driven low, both diodes 105 and 106 conduct when the output voltage becomes two diode drops lower than the reference voltage.
While the error amplifier 101 and diode 103-106 clamping stage therefor is effective, there are still some drawbacks. For example, the clamping voltage can only be selected in multiples of diode drops. Moreover, the diodes exhibit a negative temperature coefficient affecting the accuracy of the clamp voltage with temperature. Another drawback is that the absolute value of the clamp voltage is a function of the processing of the diode drops, which decreases in accuracy as the number of diodes is increased.
Another example of an amplifier which includes a clamping circuit and that does not suffer from the above-noted drawbacks is disclosed in U.S. Pat. No. 5,877,914 to Gontowski, Jr., assigned STMicroelectronics, Inc., assignee of the present invention, and which is hereby incorporated herein in its entirety by reference. The output stage of this amplifier, which operates in class AB, includes two source and the sink bipolar transistors which are serially connected between a power supply and a ground terminal. The serial connection between the emitter of the first transistor and the collector of the second transistor provides the output terminal of the output stage. Furthermore, the base terminals of the two output transistors are connected to a bias circuit and to an input transistor, which is used as the signal control element. The clamping circuit is directly connected with the base terminals of the output transistors to limit the voltage on the base terminals between a first and a second clamping voltage references.
Turning again to the error detection circuit 100 of FIG. 3, in some circumstances it may be beneficial to know when the error amplifier 101 is being driven toward positive or negative saturation. To this end, the error detection circuit 101 also includes a pair of operational amplifiers 107, 108 which are used to compare the output of the error amplifier with an internally generated reference voltage Vref. In particular, the amplifier 107 toggles high when the output of the error amplifier 101 saturates high, while the amplifier 108 toggles high when the output of the error amplifier saturates low. The outputs of the error amplifiers 107, 108 are connected via diodes 109, 110 to a first terminal of a resistor 111, a second terminal of which is connected to ground. The saturation signal is provided at the first terminal of the resistor 111.
While it may advantageously provide saturation detection, the error detection circuit 100 still suffers the above-noted drawbacks, which may render it unsuitable for certain applications. Moreover, the voltage tracking between the clamping diodes and the reference for the saturation detector, both in terms of absolute value and temperature dependency, may be less than desirable in some applications.